This invention relates to multistage transmissions having an automatic load-shift spur gear.
It is known to provide vehicles with a multistage transmission arrangement including a drive shaft connectable, by way of a first clutch, with a drive motor, at least one output shaft connectable with the drive wheels of the vehicle, a plurality of pairs of gears, each pair including one spur gear carried as a fixed gear on a gear shaft and an associated spur gear rotatably supported on another gear shaft, wherein the rotatably supported gear is connectable through gear tooth engagement or through a friction clutch with the gear shaft on which it is rotatably supported, and also including at least one additional clutch to provide a driving connection between a rotatably supported gear and its associated gear shaft.
Published European Application No. 0 367 020 discloses a five-stage transmission of the above type wherein the gears of the first four transmission stages are shifted by gear tooth engagement while the fifth transmission gear stage is shifted by frictional engagement through a controllable multiple-disk clutch. The four fixed gears of the first, second, third and fourth transmission gear stages and the rotatably supported gear of the fifth transmission gear stage are supported on the drive shaft, while the fixed gear of the first transmission gear stage is disposed adjacent to the starting clutch. In the disclosed transmission, a second clutch is capable of operation in a slip state, i.e., in partial engagement.
In that transmission, a gear stage being connected to initiate an upshift procedure is partially engaged while the original transmission gear stage is still engaged. Consequently, a branching of the driving torque occurs during the gear stage change, i.e., one part of the driving torque is still transmitted through the original gear stage, while the remaining driving torque is being supplied to the transmission output through the gear stage being connected. When the originally engaged gear stage is finally disengaged, for example, after the torque transmitted through that stage drops below a predetermined level, all of the driving torque is transmitted through the new transmission gear stage. Because of this branching of the driving torque before interruption of the original gear stage, loss of propulsive force during the upshift procedure is reduced to a minimum. Moreover, comfortable upshift procedures are made possible without being selectively influenced by the operating state of the engine. In this way, the functional operation of a multistage transmission can be controlled largely automatically. Furthermore, the highest gear stage is used as a connectable gear stage since its rotatably supported gear is connectable with the gear shaft carrying it by way of a second clutch operable in the slip state. Consequently, this arrangement always has a greater transmission ratio compared to any other gear arrangement and is connectable under load. The second clutch thus transmits torque during the shift operation and, after disengagement of the original gear stage, it provides speed reduction or speed synchronization for the next higher gear stage during upshift.
Synchronization during downshift of the transmission into engine push operation is not provided completely by engine management, since risks arise, for example, during trailer towing and running downhill, as a result of unrestricted speed-up of the engine for synchronization of the system. There is an especially great risk in push operation when, because of a nonshiftable condition, a new lower gear cannot be engaged, and the vehicle is accelerated rather than decelerated.